Unwanted particulate matter (in the form of soot, that is comprised of carbon, ash, that is comprised of inorganics, and PAH's, that are comprised of organic polynuclear hydrocarbons condensed about the carbon nuclei of the soot) is entrained in the emission stream of diesel engines. Porous ceramic and other filters that are able to capture the soot in the pores thereof are being evaluated as to their suitability for implementing the existing (1991) and future (1994 and 1998) federal and other regulations governing diesel particulate emission into the ambient atmosphere.
The soot is "sticky" and adheres quite readily to the walls defining the pores of the ceramic and other traps. With the prolongation of filtration, the soot so accumulates in the filters as to obstruct the pores, inducing thereby a back pressure that reduces the effective throughput of the filters and necessitates the cleaning of the filters.
While thermal regeneration to remove the accumulated soot from the filters is known, such as by embedding resistive filaments in the ceramic matrix that oxidize the accumulated soot when energized, because hot spots tend to be formed thereby, not only is care required to prevent degradation of the filter matrix in the locale of the hot spots but also degraded filters must be periodically monitored to ensure that they comply with the clean air emission standards. The PAH's entrained in the diesel exhaust condense at and around two-hundred (200) to four-hundred (400) degrees Centigrade. Traps which employ thermal regeneration techniques are generally located at the diesel exhaust manifold close to the engine and typically operate at temperatures well above the boiling point of the PAH's, which makes them generally unsuited to PAH emission control. Moreover, these thermally regenerated traps are prone to failure by melting and cracking of the ceramic matrix during the high-temperature regeneration periods.
In commonly assigned U.S. Pat. No. 5,013,340, entitled "Rotating Diesel Particulate Trap", incorporated herein by reference, soot is continuously removed by so rotating a particulate trap that while one sector thereof is exposed to diesel exhaust flowing in one direction another sector thereof is exposed to a counter flowing stream of low-pressure and high-velocity (high-mass) air provided either by a fan or a compressed air tank. The filter is rotatably mounted by a bearing assembly and is driven by an electric motor or belt connecting an axle to the drive shaft of the diesel engine whose exhaust is to be filtered, and rotary seals are provided to prevent cross-contamination of the exhaust and cleaning air streams.